As for a voice packet in LTE (Long Term Evolution), there are concerns about resource use efficiency and coverage because a ratio of an RTP (Real-time Transport Protocol) header, a UDP (User Datagram Protocol) header, and an IP (Internet Protocol) header to an RTP payload is high as illustrated in FIG. 8.
Therefore, PDCP (Packet Data Convergence Protocol) of LTE is expected to apply a header compression of ROHC (Robust Header Compression) method to a voice packet.
According to ROHC method, the number of bits to be actually transmitted can be reduced by transmitting only changed fields in the RTP header, the UDP header, and the IP header of each packet. ROHC method is capable of compressing an RTP header, a UDP header, and an IP header of each packet into a minimum of 3 bytes.
Hereafter, an invariable field in the RTP header, the UDP header, and the IP header is referred to as a “Static part”. Examples of the “Static part” are an SSRC (Synchronization Source), which is an identifier in RTP layer, an IP address, and others.
Meanwhile, a variable field in the RTP header, the UDP header, and the IP header is referred to as a “dynamic part”. Examples of the “dynamic part” are an “RTP timestamp”, an “RTP-Sequence Number”, a “UDP checksum”, and others.
Multiple sessions (for example, RTP sessions or RTCP (RTP Control Protocol) sessions) may be established in a single bearer. Among the multiple sessions, how many sessions the header compression can be applied to depends on capabilities of a mobile station UE and a radio base station eNB.